Powder metal technology is well known to the persons skilled in the art and generally comprises the formation of metal powders which are compacted and then subjected to an elevated temperature so as to produce a sintered product.
Conventional sintering occurs at a maximum temperature of approximately up to 1,150.degree. C. Historically the upper temperature has been limited to this temperature by sintering equipment availability. Therefore copper and nickel have traditionally been used as alloying additions when sintering has been conducted at conventional temperatures of up to 1,150.degree. C., as their oxides are easily reduced at these temperatures in a generated atmosphere, of relatively high dew point containing CO, CO.sub.2 and H.sub.2. The use of copper and nickel as an alloying material is expensive. Moreover, copper when utilized in combination with carbon as an alloying material and sintered at high temperatures causes dimensional instability and accordingly the use of same in a high temperature sintering process results in a more difficult process to control the dimensional characteristics of the desired product.
Manufacturers of metal powders utilized in powder metal technology produce prealloyed iron powders which are generally more difficult to compact into complex shapes, particularly at higher densities (&gt;7.0 g/cc). Manganese and chromium can be incorporated into prealloyed powders provided special manufacturing precautions are taken to minimize the oxygen content, for example, by oil atomization.
Notwithstanding this, these powders still have poor compressabilities compared to admixed powders.
Conventional means to increase the strength of powder metal articles use up to 8% nickel, 4% copper and 1.5% molybdenum, in prealloyed, partially prealloyed, or admixed powders. Furthermore double press double sintering can be used for high performance parts as a means of increasing part density. Conventional elements are expensive and relatively ineffective for generating mechanical properties equivalent to wrought steel products, which commonly use the more effective strengthening alloying elements manganese and chromium.
Moreover, conventional technology as disclosed in U.S. Pat. No. 2,402,120 teach pulverizing material such as mill scale to a very fine sized powder, and thereafter reducing the mill scale powder to iron powder without melting it.
Furthermore, U.S. Pat. No. 2,289,569 relates generally to powder metallurgy and more particularly to a low melting point alloy powder and to the usage of the low melting point alloy powders in the formation of sintered articles.
Yet another process is disclosed in U.S. Pat. No. 2,027,763 which relates to a process of making sintered hard metal and consists essentially of steps connected with the process in the production of hard metal. In particular, U.S. Pat. No. 2,027,763 relates to a process of making sintered hard metal which comprises producing a spray of dry, finely powdered mixture of fusible metals and a readily fusible auxiliary metal under high pressure producing a spray of adhesive agent customary for binding hard metals under high stress, and so directing the sprays that the spray of metallic powder and the spray of adhesive liquid will meet on their way to the molds, or within the latter, whereby the mold will become filled with a compact moist mass of metallic powder and finally completing the hard metallic particle thus formed by sintering.
U.S. Pat. No. 4,707,332 teaches a process for manufacturing structural parts from intermetallic phases capable of sintering by means of special additives which serve at the same time as sintering assists and increase the ductility of the finished structural product.
Moreover, U.S. Pat. No. 4,464,206 relates to a wrought powder metal process for pre-alloyed powder. In particular, U.S. Pat. No. 4,464,206 teaches a process comprising the steps of communinuting substantially non-compactible pre-alloyed metal powders so as to flatten the particles thereof heating the communinuted particles of metal powder at an elevated temperature, with the particles adhering and forming a mass during heating, crushing the mass of metal powder, compacting the crushed mass of metal powder, sintering the metal powder and hot working the metal powder into a wrought product.
Finally, coining is a process well known to those persons skilled in the art. However, a comprehensive method of precision coining of powder metal blanks is lacking. For example, U.S. Pat. No. 2,757,446 teaches a method of forming articles from metal powders which includes hot forging the article to a minimum density of 95% of the theoretical density wherein the entire change of shape of the article takes places in one direction of movement and wherein the minimum internal flow of the particles within the article is at least 5% and finally finishing the forged article.
The processes as described in the prior art above present a relatively less cost effective process to achieve the desired mechanical properties of the sintered product.
It is an object of this invention to provide an improved coining process for producing sintered articles having improved dynamic strength characteristics and an accurate method to control same, while at the same time narrowing the tolerance variability of the coined articles.
It is an aspect of this invention to provide a process of coining sintered articles of powder metal comprising blending carbon, ferro manganese and lubricant with compressible elemental iron powder, pressing said blended mixture to form said articles, high temperature sintering said articles in a reducing atmosphere and then coining said sintered articles to a final shape.
It is another aspect of this invention to provide a process of precision coining a sintered article of powder metal comprising: selecting elemental iron powder; determining the desired properties of said sintered article and selecting; a quantity of carbon; and a quantity of ferro manganese to produce an article having a composition of between 0.3% to 2.0% manganese, 0.2% to 0.85% carbon with the remainder being iron and unavoidable impurities; grinding said ferro manganese to a mean particle size of approximately 8 to 12 microns and substantially all of said ferro manganese having a particle size of less than 25 microns; introducing a lubricant while blending said carbon, and ferro manganese with said elemental iron powder; pressing said mixture to form said article; high temperature sintering said article at a temperature between 1,250.degree. C. and 1,350.degree. C. in a reducing atmosphere of 90% blended nitrogen and 10% hydrogen so as to produce said sintered article of powdered metal; then coining said sintered article to a final shape so as to narrow the tolerance variability of coined articles and substantially eliminate secondary operations.
It is another aspect of this invention to provide coined as sintered articles having a compacted and sintered mass with composition of between 0.3% to 2.0% manganese, 0.2% to 0.85% carbon, with the remainder being iron and unavoidable impurities, with a narrow dimensional tolerance variability.